1. Field of the Invention
The present invention relates to a shock absorber, and more particularly to a shock absorber for a radio control model.
2. Description of Related Art
A conventional shock absorber for a radio control model in accordance with the prior art shown in FIG. 6 is adapted to be pivotally mounted between a bracket (71) and a linkage (72) of a frame (7) of the radio control model for absorbing shocks from the supporting surface.
Generally speaking, the shock absorber usually contains oil to promote the damping modulus of absorbing shock. However, the radio control model usually operated in a high speed such that the shock absorber must has a great damping modulus during absorbing shock and a small damping modulus during restituting.
With reference to FIG. 6, the conventional shock absorber comprises a cylinder (81) for containing oil and having an upper end pivotally connected to the bracket (71). A piston (82) is reciprocally movably received in the cylinder (81). The piston (82) has a diameter smaller than an inner diameter of the cylinder (81) such that a gap (A) is formed between an inner periphery of the cylinder (81) and the piston (82). The piston (82) has multiple through holes (821) longitudinally defined in the piston (82) and a plastic film (83) secured on a top surface of the piston (82) for selectively closing the through holes (821) in the piston (82). A shaft (84) has a first end securely connected to a bottom of the piston (82), and a second end extending out of the cylinder (81) and pivotally mounted to the linkage (72) that is connected to a wheel axle of the radio control model. A spring (85) is compressively mounted around the conventional shock absorber.
With reference to FIG. 7, the piston (82) is upwardly moved to compress the oil in the cylinder (81) by the shaft (84) and the spring (85) is compressed when the linkage (72) is lifted duo to a rough supporting surface, thereby the plastic film (83) securely abuts the piston (82) for closing the through holes (821) due to a pressure of compressed oil in the cylinder (81) such that the oil only flows and passes through the gap (A). Consequently, the conventional shock absorber has a greater damping modulus for absorbing shock.
With reference to FIG. 8, the plastic film (83) is upwardly pushed by the oil of backflow and the through hole (821) is opened during the piston (82) moving to the original condition of the shock absorber due the pressure of the compressed oil and the restitution force of the spring (85) when the shock disappears. Consequently, the damping modulus of the shock absorber is smaller than that of the shock absorber when the plastic film (83) closing the through hole (821) because the oil of backflow can flow through the through hole (821) in the piston (82) and the gap (A) such that the piston (82) quickly moves back to the original position.
As described above, the plastic film (83) is transformed to open the through hole (821) in the piston (82) for changing the damping modulus of the shock absorber. However, the plastic film (83) will be deteriorated after being dipped in oil for a long time. Furthermore, the piston (82) is continuously and reciprocally moved in the cylinder (81) so that the oil in the cylinder (81) will become high. The plastic film (83) easily, has a permanent deformation even broken due to the high temperature of the oil in the cylinder (81). Then the plastic film (83) should not effectively cover and close the through hole (821), thereby the conventional shock absorber cannot effectively absorb the shock when the radio control model is operated.
The present invention has arisen to mitigate and/or obviate the disadvantages of the conventional shock absorber for a radio control model.
The main objective of the present invention is to provide an improved shock absorber for a radio control model.
To achieve the objective, the shock absorber in accordance with the present invention comprises a cylinder and a piston reciprocally received in the cylinder. The piston has an annular groove defined in the periphery thereof and dividing the piston into an upper portion and a lower portion. At least two first holes are defined in the upper portion and communicates with the annular groove. At least one second hole is defined in the lower portion and communicates with the annular groove.
At least one of the second hole co-axially aligns with a corresponding one of the at least two first holes and the number of the first hole is greater than that of the second hole. At least one stopper is received in the piston between the first hole and the second hole that align with each other for selectively closing the first hole and the second hole.